Quenched and tempered steel plates (hereinafter also referred to as “QT steel plates”) have high strengths and good toughness, have satisfactory weldability, and have therefore been widely used in welded structures such as bridges, high-rise buildings, ships, and tanks. With increasing sizes of welded structures in recent designing, the QT steel plates are required to have higher strengths (for example, a yield strength of 415 MPa or more and a tensile strength of 620 MPa or more).
Steel plates should not only have high strengths but also exhibit good drop weight properties which are indices of brittle fracture properties. However, with increasing strengths and thicknesses required of steel plates in present circumstances, it is difficult for the steel plates to have good drop weight properties.
Patent Literature (PTL) 1 discloses a technique as a possible solution to improve drop weight properties. According to this technique, a phosphorus content is minimized to induce grain boundary strengthening (crystal stressing), nitrogen is added in a predetermined amount to induce grain refining effects, and chromium is added to improve toughness. A steel sheet obtained according to the technique, however, has a nil-ductility transition temperature (NDT) of at most about −50° C. and does not meet the recently required properties. The nil-ductility transition temperature is an index of drop weight properties.
PTL 2 proposes a technique of performing low-temperature rolling to form fine ferrite grains to thereby provide good drop weight properties. This technique, however, fails to give high strengths and therefore fails to provide both good drop weight properties and high strengths compatibly.
PTL 3 proposes a technique of performing quenching with a roller quench system to form fine ferrite grains while suppressing the formation of bainite, so as to provide good drop weight properties. Even this technique, however, fails to give high strengths and fails to provide both good drop weight properties and high strengths compatibly.